Petrology, thermobarometry and U-Pb dating on the rocks of Delbar complex, Biarjmand area (southeast of Shahrood)

The Delbar metamorphic complex is located 130 km southeast of Shahrood (Biarjmand area) in the northern margin of the Central Iran. This complex consists of four groups include metapelitic، metapsammitic، metacarbonatic and metabasitic compositions. Field evidences indicate the protolith of the complex were shale and sandstone (psammite and graywacke) alternation with carbonate layers in the upper parts. The complex crosscut by numerous diabasic dikes with late Neoprotrozoic- early Cambrian age which evolved to the foliated metabasites such as amphibole schist، amphibolite and garnet amphibolite. The sedimentary sequence in which diabasic dikes intruded subjected by intermediate P-T regional metamorphism (Barrovian type). The results of thermometry based on the varied calibrations of garnet-biotite pair and barometry through GBPQ method demonstrate 468-498 ˚C and 6. 4-7. 2 Kbar for micaschists and 645-706 ˚C and 8. 4-13. 2 Kbar for gneisses indicating greenschist for micashists and upper amphibolite facies for gneisses respectively. The increase of temperature and pressure confirms the prograde regional metamorphism event up to acheive the melting point and the partial melting initiation. The diverse migmatite structures generated in the metapelitic and metapsamitic rocks from felsic meltsexposed as the anatectic granites and leucogranites. The results of U-Pb age dating indicate 541±4. 7 Ma to 547±9. 9 Ma (late Neoprotrozoic- early Cambrian) age range. The regional metamorphism event in the Delbar complex is related to the Pan-African orogeny according to the obtained age data and with comparsion to the other similar metamorphic complexes and granitoids in the Central Iran.